Exhaust system for a lean-burn ic engine

ABSTRACT

An exhaust system for a lean-burn internal combustion engine comprises a filter ( 30, 36 ) for particulate matter and means ( 24 ) for generating an oxidant more active than molecular oxygen for combusting particulate matter disposed on the filter ( 30, 36 ), wherein the filter ( 30, 36 ) comprises a mass of elongate flat, narrow strip metal.

The present invention relates to an exhaust system for a lean-burninternal combustion engine, and in particular to a system for treating asoot-containing gas.

Whereas filtering soot from engine exhaust gas by ceramic wall-flowfilters has become well established, use of metal-based filters is lessso. Metal-based filters were disclosed inter alia in U.S. Pat. No.4,270,936 and U.S. Pat. No. 4,902,487 and in SAE papers 820184 (Enga etal.) and 890404 (Cooper, Thoss). They reached an advanced stage ofdevelopment in Johnson Matthey's ‘Catalytic Trap Oxidiser’ (‘CTO’), butappear not to have successfully competed with wall-flow filters in thecommercial market. We have recently identified systems in whichmetal-based filters can be used with advantage.

According to the invention there is provided an exhaust system for alean-burn internal combustion engine, which engine comprising an exhaustgas treatment system comprising a soot filter packed with a mass ofelongate flat, narrow strip metal and means for generating an oxidantmore active than molecular oxygen (O₂) for combusting soot collected onthe filter.

By “elongate” herein, we mean relative to the width of the flat of theflat strip metal. The term “narrow” is to be interpreted accordingly.

By “packed” herein, we also mean “compacted” and “compressed”.

The exhaust gas from such an engine typically contains the gaseouscomponents soot (or particulate matter (PM)) unburned hydrocarbons (HC),carbon monoxide (CO), nitrogen oxides (NO_(x)), carbon dioxide (CO₂),water vapour (H₂O), O₂ and nitrogen (N₂). The means to remove bycombustion the soot collected on the filter preferably operatescontinuously. The oxidant more active than O₂ is for example ozoneand/or plasma, most conveniently NO₂. Such NO₂ is preferably provided,at least in part, by catalytic oxidation of the NO component of the NOxe.g. on a NO-oxidation catalyst e.g. platinum supported on particulatealumina upstream of the filter. Such catalyst may be supported on apacked flat, narrow strip metal substrate, conveniently of the type usedin the filter, but at a lower packing density, to permit passage of sootparticles.

Alternatively or additionally the filter packing may carry a layercatalytic for soot oxidation, possibly by a mechanism involvingoxidation of NO to NO₂. Such soot oxidation catalysts include supportedplatinum group metals, such as platinum on alumina and/or base metalssuch as La/Cs/V₂O₅. If the engine-out NOx available in the gas isinsufficient to combust the soot continuously, more may be introduced,e.g. by introduction of plasma or NOx or nitric acid, possibly as gasproduced by oxidation of ammonia on-vehicle.

The ozone and/or plasma may be generated by suitable means such as asource of UV light and/or a corona discharge device. It is to beunderstood that plasma and/or ozone is capable of oxidising NO to NO₂.In one embodiment, the exhaust system comprises both means forgenerating ozone and/or plasma and as NO oxidation catalyst.

The external structure of the filter may have features providingoperational advantages. For example, it may be formed as a monolitheasily inserted into or withdrawn from a reactor shell. Whethermonolithic or not, it may be disposed as a cartridge in an outer shell,easily insertable or withdrawable. It may be capable of electricalconduction as a whole, thus permitting electric heating at cold start.Such electrical conduction may be used in constructing a monolith, byeffecting local welding between adjacent strips; if the filter is to bedisposed in an outer shell. It may contain an axial metal rod to actduring such welding as one electrode, the shell acting as the other.Further external features are mentioned below.

The metal of the filter should be capable of withstanding the exhausttreatment process conditions. Since the filter can be replaceable moreeasily than a ceramic filter, and its material can be recovered forre-use, the use-life of the filter need not be as long as for a ceramicfilter. It is possible to envisage replacing the filter at the normalservice interval of a vehicle.

Typically the metal is a corrosion resistant iron alloy. Typical alloyscontain nickel and chromium and minor constituents as in Type 300 orType 400 stainless steels. Which is used may depend on whether theexhaust gas treatment system is required to operate temporarily in richconditions, in which some stainless steels are unstable. For a widevariety of exhaust compositions a preferred iron alloy contains at least11.5% Cr, 4% Al and 0.02-0.25% minor constituents such as rare earth,zirconium or hafnium. The metal in a filter may be a mixture ofdifferent compositions, possibly including a component providingelectrical conduction bridges or a welding function.

The filter may have, wholly or domain-wise, a regular structure, forexample coiled, woven or knitted. The metal strip of the filter may befor example up to 2, especially in the range 0.1 to 0.5 mm, wide. Itshould be thick enough to afford mechanical strength in the conditionsin which it is to be used. Typically its thickness is in the range 0.2to 0.8 of its width. Suitably its geometric surface area per unit lengthis in the range 1.2 to 1.5 times that of the same weight of metal incircular cross-section. It is suitably the product of flatteningcircular-section wire. The metal in a filter unit may be a mixture ofstrip dimensions and may include circular-section wire as unflattenedinterlengths or as added sub-units. In one embodiment, the flat narrow,strip metal is of flattened wire.

The level of packing can be chosen to provide a desired level offiltration and/or backpressure in the system, and can depend on thewidth and depth of the flat metal strip. However, we believe thatgenerally a range of packing density of from 2.5 to 30% v/v, such as 5to 15% v/v can provide the desired result. We have used a packingdensity of 10% with advantage.

A catalytic coating on the filter typically comprises a washcoat ofoxide such as alumina with possibly rare earth and an active materialespecially Pt or Pd or oxides of Cs and V. The coating may containperovskite. If catalytic oxidation of NO is used, the catalyst typicallycomprises Pt and/or Pd on such a washcoat. If ozone is used, thegenerator thereof may be for example a corona discharge tube throughwhich air passes between two electrodes kept at a large potentialdifference; or may comprise a high-energy lamp. If plasma is used, theplasma generator may operate for example by corona discharge, surfaceplasma discharge or dielectric barrier discharge or comprise adielectric packed bed or electron beam reactor. It may be enhanced byelectromagnetic radiation such as microwave radiation. The generator maytreat air or the whole of the exhaust gas or part of such gas before orafter treatment.

The size of the filter(s) relative to the engine and any arrangements tointroduce additional oxidant more active than O₂ may be the subject ofdesign features. In the simplest case filter capacity is large enough sothat soot is combusted continuously by the oxidant, that is, with anyaccumulation during slow running being quickly removed in periods offast running; the overall trend being continuous combustion. A lessexpensive filter capacity is sized to accommodate larger accumulationsof soot, sufficient to increase pressure-drop significantly before thenext period of fast running. Such filter(s) preferably includes abypass, the pressure-drop through which is equal to the design maximumtolerated pressure-drop. The bypass avoids engine stalling or low powerthat would result from excessive pressure-drop, but permits some sootemission to atmosphere. To cope with such soot emission a second stagesuch as a filter or impingement collector and/or an oxidation catalystmay be provided downstream of the bypass. The bypass, without or withsecond stage filter and/or oxidation catalyst, may be part of the filtercartridge.

The direction of gas flow through the filter and/or (if used) oxidationcatalyst can be or have a component linear or transverse to the generalflow direction. Transverse flow may be for example symmetrical,especially inwards to an outflow header axial in a cylindrical filter,or to a plenum in an oval-section or rectangular filter. Alternativelyone-way cross-flow may be provided.

In a further elaboration of the process and system a succession offilter elements presents to the gas a different soot-treating capacity,for example collecting smaller and smaller particles, and/or providinggraded catalytic environments. Preferably gas flow in the filterelement(s) at the inlet of the succession is, or has a component,transverse to the general direction of flow. If the process and systemincludes subjecting soot to oxidant more active than molecular oxygen,successive filter elements may alternate with oxidation catalyst and/orwith means to provide plasma or ozone. In such succession downstreamfilter element(s) and (if used) oxidation catalyst(s) may be ceramic.

A particularly useful system comprises, in downstreamward order, aplurality of metal-based filters for successively trapping smaller andsmaller particles and, optionally, at least one wall-flow filter fortrapping yet smaller particles. In this system the pores of thewall-flow filter can be smaller than in single-stage wall-flow trapping,because the preceding metal-based filters have removed the largerparticles that may have blinded or blocked, i.e. reduced the gas flowthrough, them. Any or all of the filters may be catalysed.

Instead or in addition a distinct NO-oxidation catalyst may be disposedupstream of at least the first filter. Such catalysis on and/or betweenfilters can have the effect of restoring the NO₂ content, which may havehad been decreased by reaction with soot in the preceding filter. Thefilters and, if present, catalysts, may be assembled as a single unitwithin a cartridge. Such NO-oxidation catalysts can be supported on aflow through substrate e.g. a ceramic or metal substrate.

According to a further aspect, the invention provides a system accordingto the invention wherein the oxidant more active than O₂ is at least oneof ozone, plasma or NO₂.

The exhaust treatment system may include other integers as used orproposed, for example a three-way catalyst (TWC), nitrogen oxide (NOx)trap+regeneration means, selective catalytic reduction (SCR) e.g. usinghydrocarbon or ammonia as reductant, lean-NOx catalysis, a sulfur oxides(SOx) trap regenerable or disposable. The engine and system may includecontrol gear, in use, for controlling the operation of the exhaustsystem to reduce emissions and on-board diagnosis gear as usual oradapted to novel features of the invention.

The lean-burn engine may be any engine currently or potentiallyproducing a soot-containing engine. For example the engine may be acompression ignition engine, such as diesel engine, or a spark ignitionengine such as a lean burn gasoline, e.g. gasoline direct injection(GDI™), engine. It may have exhaust gas recirculation (EGR). It may befor light or heavy duty. To provide for the low SO₂ content of theexhaust gas, the S content of the fuel used should be less than 500,especially less than 50 ppm w/w S. Low sulfur fuelling and lubricationgiving exhaust gas of less than 20 ppm SO₂ is preferred.

In order that the invention may be more fully understood embodimentswhereof will be described with reference to the accompanying drawingsand by reference to the illustrative Example, wherein:

FIG. 1 shows in schematic section a diesel engine with an exhaustsystem;

FIG. 2 is a trace showing exhaust gas aftertreatment component inlettemperature and outlet temperature in the exhaust system of a vehicleagainst time, also showing vehicle speed;

FIG. 3 is a schematic sectional view through an exhaust gas treatmentsystem component for use in the present invention;

FIG. 4 is a bar chart showing particulate mass measured over a drivecycle for exhaust gas treatment systems 1, 2 and 3; and

FIG. 5 shows modal (second by second continuous) analysis of tailpipeNO₂ from an exhaust system comprising systems 1, 2 and 3.

Referring to FIG. 1, item 10 indicates a 4-cylinder diesel engine havingair inlet 12 and fuel inlet 14 fed with hydrocarbon of 5 ppm sulfurcontent at an air/fuel weight ratio of about 30 for steady operation butvariable as routinely practised. The engine exhaust 16 is fed to acylindrical treatment reactor indicated generally by 18 and havinginsulated internal walls 20. Fitting snugly within walls 20 is filtercartridge 22. At the inlet end of cartridge 22 and occupying its wholediameter is catalyst bed 24, packed with knitted 310 stainless steelflattened wire 0.33 mm wide and 0.2 mm thick to 6% solid by volume,carrying an alumina washcoat and Pt at 70-100 (1.98-2.83 gm⁻³), possiblyup to 300 (8.50 gm⁻³), g/ft³ of bed volume, giving low-temperaturelight-off. The next downstream zone of cartridge 22 is occupied byannular feed channel 28 surrounding first filter 30 packed with the sameflattened wire as in bed 24 but at 12% volume by volume and carrying awashcoat and soot oxidation catalyst. Filter 30 provides axial-inwardgas flow to outlet 32. Feed channel 28 terminates longitudinally inbypass 34, the function of which will be explained below. The nextdownstream zone is second filter 36, providing longitudinal gas flow.Axial to filter 36 is metal rod 38, the function of which is to beexplained below. Filter 36 is packed with the same flattened wire asused in bed 24 but at 16% volume by volume and carrying a soot oxidationcatalyst e.g. La/Cs/V₂O₅. Filter 30 and/or 36 may be rigidified, at thetime of construction, by electric internal spot welding usingrespectively the axial outlet or axial rod 38 as one electrode and theouter boundary member as the other electrode Surrounding filter 36 isbypass channel 40. Bypasses 34 and 40 are shown shaded to indicate thepossible inclusion of flow-obstructing material to provide balancing ofpressure-drop with that of the filter when the filter is partlysoot-bearing. Instead of or in addition to the soot oxidation catalyston filter 36, there may be a second oxidation catalyst, similar to 24,between the filters.

In operation, NO in the exhaust gas entering bed 24 is largely oxidisedto NO₂. Soot in the gas passes through bed 24 and is held on filter 30where it is oxidised by the NO₂ to CO. If filter 30 is under-designed oran engine upset produces extra soot, soot accumulates in it andobstructs gas flow through it. At a design, i.e. pre-determined, levelof pressure-drop due to such obstruction, bypass 34 permits gas to passthrough the exhaust system, so that engine operation can continue untilsoot-oxidising conditions return or remedial action is taken. Likewise,if soot accumulates in filter 36 to a design level, bypass channel 40permits gas to pass.

Filters as 30 and 36, and possibly others in succession, providesuccessively increasing geometric surface per unit volume, to trap finerparticles or bypassed particles. Such successive filters need notinclude a bypass, if the entering concentration of soot is sufficientlyless than in the first filtering stage. Such further stage(s) mayinclude oxidation catalyst as mentioned above to restore the content ofNO₂ following reduction by soot on the preceding filter.

EXAMPLE

A 2.5 litre Audi TDI vehicle certified for European Stage 2 legislativerequirements, and fuelled with 50 ppm sulphur containing diesel fuel,was fitted with a flow through ceramic monolith 5.66 in (144 mm)diameter and 9 in (225 mm) long with a cell density of 400 cells persquare inch (cpsi) (62 cells cm⁻²) and 6 mil (thousandths of an inch)(0.15 mm) wall thickness. The vehicle was placed on a standard chassisrolling road dynamometer and, after 20 seconds idling was accelerated to120 kph in 100 seconds and maintained at this speed for the remainder ofthe test. After a further 300-400 seconds the inlet temperature to thecatalyst system attained a stable temperature of 330-350° C. The vehiclewas then run for a period of 20 minutes at this temperature (FIG. 2).During the 20 minute period particulate was collected on two sets offilter papers by the standard method (one set for each 10 minute period)to enable an average particulate weight for the test to be calculated.During the same 20 minute period Nitrogen Oxides (NO and NO₂) weremeasured in the feed gas to the monolith, and the tail pipe gas afterthe monolith by chemiluminescent analysis and Fourier Transform InfraRed (FTIR) respectively. This was labelled System 1.

The flow through monolith was removed from the vehicle and replaced by a5.66 in (144 mm) diameter and 4 in (100 mm) long flow through monolithof the same cell density and wall thickness coated with a platinumcatalyst at 75 gft⁻³ (2.6 g litre⁻¹) followed by a bare flow throughmonolith 5.66 in (144 mm) diameter and 4 in (100 mm) long. The identicaltest cycle was conducted and the measurements repeated. This waslabelled System 2.

The oxidation catalyst was replaced by one 5.66 (144 mm) diameter and 3in (76 mm) long flow through monolith coated with platinum at 75 g ft⁻³(2.6 g litre⁻¹), and the bare monolith was replaced by a particulatetrap of novel design. This consisted of a packed bed of knitted,stainless steel flattened wire, 0.10 mm wide and 0.05 mm thick,occupying the whole diameter of the rear face of the catalyst andabutting against it. The total length of the wire bed was 4 in (100 mm)and the packing density was 10% v/v. This was followed by a 5.66 in (144mm) diameter and lin (25.4 mm) long flow through monolith coated withplatinum at 75 g ft⁻³ (2.6 g litre⁻¹) (“catalyst slice”) abutted againstthe rear face of the knitted wire substrate. This catalyst was followedin turn by a second knitted wire substrate and a bare monolith, havingthe same dimensions as the preceding first knitted wire substrate andcatalyst slice. The arrangement is shown in FIG. 3. Therefore the totalvolume and aspect ratio of catalysed flow through monolith was the sameas that of System 2. This last system was labelled System 3. Theidentical drive cycle and measurements were repeated as for Systems 1and 2.

FIG. 4 summarises the particulate mass measured over the drive cycle forthe three systems. Thus a lowering of particulate from the “baseline”System 1 by the addition of oxidation catalyst is obtained in System 2and a further improvement with the addition of the packed wire bed whichretains a proportion of the soot allowing NO₂ exiting the firstcatalyst, to react with it. It can be seen that the conversionefficiency of System 2 relative to system 1 is 13%, whereas theconversion efficiency of System 3 relative to System 1 is 38%.

FIG. 5 shows modal (second by second continuous) analysis of NO₂ at thetailpipe downstream of the three systems. System 1, with a bare monolithhas very low NO₂ emissions similar to those from the engine. Higher NO₂concentrations are measured after System 2 because NO is oxidised overthe catalyst, but there is only a small amount of reaction with soot. InSystem 3 NO₂ formed over the first oxidation catalyst reacts with sootcollected in the first filter bed. The NO not oxidised over the firstcatalyst passes through the first filter bed and is oxidised to NO₂ overthe second catalyst. There may also be some re-oxidation of NO formedfrom reaction between NO₂+C→NO+CO. This NO₂ together with any NO₂ notreacted in the first filter reacts with soot collected in the secondfilter bed resulting in lower soot and NO₂ tailpipe levels, compared toSystem 2. Turbulent flow, initiated in the gas stream by its passagethrough the filter bed, enhances the reactions of NO₂ with the soot andthe oxidation of NO over the second catalyst.

1. An exhaust system for a lean-burn internal combustion enginecomprising a soot filter packed with a mass of elongate, flat, narrowstrip metal and a catalyst located upstream of the filter for oxidisingNO to NO₂ for combusting soot collected on the filter in NO₂, whereinthe catalyst is supported on a metal substrate of the type used in thefilter having a lower packing density, to permit passage of sootparticles.
 2. A system according to claim 1, comprising, in order fromupstream to downstream, a plurality of metal-based filters adaptedsuccessively to trap smaller and smaller particles.
 3. A systemaccording to claim 2, comprising at least one wall flow filter fortrapping yet smaller particles.
 4. A system according to claim 2,comprising a flow-through monolith between the or each pair ofmetal-based filters.
 5. A system according to claim 4, wherein the oreach flow-through monolith comprises a NO oxidation catalyst, whereby torestore the NO₂ content, which had been decreased by reaction with sootin the preceding filter.
 6. A system according to claim 1, wherein thefilter capacity is sufficient to allow the soot to be combustedcontinuously by the oxidant.
 7. A system according to claim 1, whereinthe filter capacity is sized for accumulations of soot sufficient toincrease pressure-drop significantly before the next period of fastrunning and the system includes a bypass, wherein the pressure-dropthrough which is equal to the design maximum tolerated pressure-dropthrough the filter, whereby to avoid engine stalling.
 8. A systemaccording to claim 7, comprising means to limit soot emission toatmosphere located downstream of the bypass, which means being selectedfrom the group consisting of a filter, an impingement collector and anoxidation catalyst.
 9. A system according to claim 1, wherein the filtercomprises a regular coiled, woven or knitted structure.
 10. A systemaccording to claim 1, wherein the metal of the filter is Type 300 orType 400 stainless steel.
 11. A system according to claim 1, wherein themetal from which the filter is made comprises an iron alloy containingat least 11.5% Cr, 4% Al and 0.02-0.25% minor constituents such as rareearth, zirconium or hafnium.
 12. A system according to claim 1, whereinthe width of the metal strip of the filter is up to 2 mm and itsthickness is 0.2 to 0.8 times its width.
 13. A system according to claim12, wherein the flat, narrow strip metal is a flattened wire.
 14. Asystem according to claim 1, wherein the filter packing carries a layercatalytic for soot oxidation.
 15. A system according to claim 14,wherein the catalytic layer comprising a washcoat and a componentselected from the group consisting of Pt and oxides of Cs and V.
 16. Asystem according to claim 1, comprising means for generating a componentfor combusting soot collected on the filter selected from the groupconsisting of ozone and plasma.
 17. An internal combustion enginecomprising an exhaust system according to claim
 1. 18. A diesel engineaccording to claim
 17. 19. A system according to claim 3, comprising aflow through-monolith between the or each pair of metal-based filters.20. A system according to claim 19, wherein the or each flow-throughmonolith comprises a NO oxidation catalyst, whereby to restore the NO₂content, which had been decreased by reaction with soot in the precedingfilter.
 21. A system according to claim 12, wherein the width of themetal strip is in the range 0.1 to 0.5 mm.